Conventional technologies for designing and manufacturing tunable filters for telecommunication systems implemented with fiber optics are still faced with significant technical challenges. Specifically, in the field of fiber optics communications implemented with dense wavelength division multiplexing (DWDM) for transmitting optical signals in multiple channels in the optic fibers, the tunable filter is one of the most important components. In order to achieve high performance low cost and space savings with miniaturized components, a tunable filter is required to satisfy very demanding specifications. Under most circumstances for a DWDM system, a tunable filter is required to have low insertion loss, narrow bandwidth, flat passband, large tuning range, fast tuning speed, compact size and low driving power. In order to satisfy these very demanding requirements, only thin film filters are currently implemented with fixed filtering wavelength. Due to the technical difficulties in meeting these highly demanding system requirements, implementations of tunable filters are still not achievable in most cases even that a tunable filter can provide significant advantages of more operational flexibility, simplified system configurations and significant cost savings. Even with these technical challenges, there is still a great demand to provide a tunable filter to overcome these difficulties because the tunable filter is a critical component in the dynamically configurable routers, optical channel monitoring, and tunable lasers.
In the recent past, developments of the tunable filters to offer such performances have been focused mostly on acousto-optical tunable filter. Also, there are efforts to develop mechanical tunable filters by applying motor or micro electromechanical switch (MEMS), and liquid crystal (LD) based tunable filters. Additional efforts are also devoted to individual AO tunable filters, individual LD tunable filters and diffraction grating tunable filters with mechanical tuning.
Generally, an acousto-optic tunable filter (AOTF) has advantages of fast tuning speed, large tuning range. There were attempts to improve the performance of AOTF to provide narrower spectral bandwidth. However, narrower bandwidth often is achieved in the expense of large device size, high electrical power consumption and complexities of internal structure, thus raise cost and reliability concern and limits its practical applications in the telecommunication systems.
On the other hand, a diffractive grating has advantage of low insertion loss, narrow bandwidth, and excellent thermal stability. Different wavelength tuning mechanisms have been implemented in the incident beam port and mostly by means of thermal and mechanical tuning. Thermal tuning is slow and can provide only limited tuning range due to its impractical temperature elevation. Thermal tuning also limits the tuning resolution. Mechanical tuning is very sensitive to shock and vibration and is a very unstable method for telecom applications.
A fixed diffraction grating combined with an electrically tuned beam-directing device will ideally take advantages of both tuning mechanisms. In one of the methods U.S. Pat. No. 5,946,128, it describes a tunable filter with an AO deflecting filter and a diffraction grating. However, its AO deflecting filter is essentially a non-collinear AOTF. The acousto-optical filter only filters a selective band of wavelengths and therefore requires a large beam aperture into the AOTF. It then relies on the diffraction grating to further separates the incoming wavelengths to multiple ports. For very narrow band optical power monitoring applications where filter bandwidth is <0.1 nm and still smaller preferred, the requirements for the AO crystal is in the order of 20 mm cubes and will consume watts of electrical power along with large size making it very impractical for telecom applications. In addition, the filter shape is typically SINC-like having none-flat pass-band inducing distortions in DWDM signal filtering.
FIG. 1 illustrates a conventional device that combines a diffraction grating and an acousto-optic beam deflector to form a so-called grating assisted acousto-optic tunable filter. This device claims to have narrow bandwidth for selected wavelengths, down to 0.13 nm, while maintaining the rapid tenability of ˜10 microseconds. A tuning range of up to 52 nm is achieved to cover the EDFA working range around 1550 nm. However, the device has following limitations.    1. In order to have a high spectral resolution from the diffraction grating, the collimated beam from lens 2, going through acousto-optic beam deflector 3, to the grating 4, needs to be large. For example, in the U.S. Pat. No. 5,946,128, a 6-mm diameter beam results in a 0.2-nm spectral bandwidth. A further increase in spectral resolution would require even larger beam size. However, a large beam size on AOTF demands a large size of the acousto-optic material, which puts serious limitations on practical applications: high RF power to drive the crystal and then high cooling capacity needed; large crystal volume leading to high cost of manufacturing and unfit to limited space in telecom applications.    2. Since the diffraction grating determines the spectral bandwidth, the filter spectral shape is essentially proportional to
                                          {                                          sin                ⁡                                  (                                      N                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                    d                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                                          φ                      /                      λ                                                        )                                                            sin                ⁡                                  (                                      π                    ⁢                                                                                  ⁢                    d                    ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                                          φ                      /                      λ                                                        )                                                      }                    2                ,                            (        1        )                            where N is the total number of grooves of the grating, d is the grating constant, and φ is the diffraction angle. This none-flat passband is not preferred in DWDM applications.            3. The tuning speed is related to the beam size on the acousto-optic beam deflector. In order to keep tuning time at less than 10 microseconds, the aperture of acousto-optic beam deflector 3 needs to be 6 mm or less. On the other hand, the high spectral resolution from the diffraction grating requires a large beam size on the grating. Therefore, there is an inherent trade-off between tuning speed and spectral resolution in the prior art design.    4. Wavelength tuning range limitation is related to RF tuning range and can become very limiting in practical implementations.
For these reasons, there is still a need in the art to provide an improved design and manufacture process to provide a high resolution narrow linewidth tunable filter tunable with low power consumptions and suitable for assembled into a small package for practical applications in a typical telecommunication systems implemented with DWDM technologies. It is desirable that the new and improved tunable filter can be produced with high level of optical performance characteristics and low cost.